Turbochargers are widely used in internal combustion engines to increase engine power and efficiency, particularly in the large diesel engines of highway trucks and marine engines. It is particularly advantageous in these types of engines to use turbochargers that are designed to provide a very high pressure ratio (the differential pressure across the compressor), compared, for example, to turbochargers typically used in smaller passenger engines. The use of a turbocharger permits selection of a power plant that develops a required number of horsepower from a smaller and lighter engine. The use of a lighter engine has the desirable effect of decreasing the total mass of the vehicle, and the reduced envelope of a smaller engine may be used to enhance the aerodynamics of the vehicle and thus reduce drag. Both of these factors enhance fuel economy and increase performance. In addition, the use of a turbocharger permits more complete combustion of the fuel delivered to the engine, which reduces hydrocarbon and NOx emissions, thereby contributing to the highly desirable goal of a cleaner atmosphere. Recently, turbochargers have also become increasingly popular for use in smaller, passenger car engines.
Turbochargers generally comprise a turbine housing that directs exhaust gases from an exhaust inlet to an exhaust outlet across a turbine rotor. The turbine rotor drives a shaft journaled in a bearing housing section. A compressor rotor is driven on the other end of the shaft, which provides high-velocity air to a diffuser. The general design and function of turbochargers is described in detail in the prior art, for example, U.S. Pat. Nos. 4,705,463; 5,399,064; and 6,164,931, the disclosures of which are incorporated herein in their respective entireties by reference.
In a radial-flow or centrifugal turbocharger, the compressor rotor receives and pressurizes the inlet gas. The compressor rotor discharges the gas with high tangential and radial components of velocity. The gas flows over a diffuser, in which the kinetic energy, or velocity head, is converted to a static pressure by deceleration or diffusion of the flow, and the temperature and pressure of the gas are increased. The increased temperature improves combustion efficiency, while the increased static pressure at the engine inlet may be used to increase the mass of air/fuel mixture in the cylinder, and/or to improve the air:fuel ratio.
The design of turbocharger compressors is a highly refined art. The shape, curvature, and surface finish of the compressor rotor, compressor housing, and diffuser are designed to produce maximum pressure boost across the desired range of operating conditions. When very high pressure ratios are required, as in the case of large commercial diesel engines, vaned diffusers are generally preferred over vaneless diffusers because they provide a higher maximum pressure ratio and increased efficiency, albeit frequently at the cost of a reduced map width, as depicted on a compressor map well known in the art as showing the relationship between pressure ratio and volume or mass flow rate.
The design of the diffuser is critical to achieving efficient turbocharger operation over a usefully wide range of engine operating conditions. While it is relatively straightforward to design a diffuser for constant inlet and outlet conditions, variations in the flow rate, and the nature of the flow increase the difficulty of providing a satisfactory diffuser for a useful range of operating conditions. Design parameters for compressors have been refined to the extent that a change of the order of 0.5-1.0% in efficiency is significant within the art. A general rule of thumb is that each one percent improvement in the efficiency of the compressor produces a one-third percent improvement in the brake specific fuel consumption (BSFC) of a diesel engine.
The vanes of a vaned diffuser define channels into which high velocity gas from the compressor is received, and through which the gas is decelerated in order to convert its kinetic energy into a static pressure. Circumferentially spaced guide vanes provide passages that expand radially in area to diffuse the flow. Because the gas flow characteristics vary with operating conditions, a high-quality surface finish and the angle of attack of the vanes are critical parameters in the design of an efficient vaned diffuser. The cross-section and shape of the vanes of a vaned diffuser are also important design parameters. Wedge-shaped, straight-sided blades, referred to as straight island type, provide a high pressure ratio and high efficiency at the expense of operating range. On the other hand, curvilinear cross-section blades permits flow straightening in the diffuser, as disclosed in U.S. Pat. No. 2,844,001. Vanes that have an aerofoil cross-section are also known in the art, as are vanes that are divided along their length, in which each portion is optionally radially offset. For smooth and uniform exit flow from the diffuser, thin edged vanes are desirable. The width of the diffuser is also an important design parameter. Therefore, in order to implement the best designs and to reap their intended benefits, it is necessary that the vanes of the diffuser are manufactured to very close tolerances. The typical design parameters for vaned diffusers are disclosed in the prior art, for example, in U.S. Pat. Nos. 2,844,001; 1,047,663; 3,936,223; 3,997,281; 3,719,430; 4,815,935; and 5,277,541, the disclosures of which are incorporated herein in their entireties by reference.
Vaned diffusers are constructed as a separate component of the compressor housing, and are typically shaped in the form of an annular ring designed to fit against a backplate or axial wall surface. Clearance gaps exist between the top of the vanes and the opposing diffuser wall due to machining tolerances. Additionally, the compressor housing is known to expand or move axially away from the backplate wall under turbocharger operating temperatures and pressures, which can further increase gaps. The contemporary vaned diffusers utilize an annular wave spring in order to provide constant pressure loading during compressor operating temperatures and pressures to ensure that the vanes on the diffuser ring remain in contact with the opposing wall, such as the backplate wall. An O-ring sealing structure must then be employed to reduce or eliminate any losses due to gaps formed between the diffuser ring and the compressor housing or center housing.
An example of a contemporary vaned diffuser is shown in U.S. Pat. No. 4,354,802 to Nishida. The Nishida vaned diffuser, as shown in FIG. 1, does not use either a biasing means or a sealing structure. The turbocharger 1 has a plurality of compressor vanes 2 positioned along a diffuser ring 4 within a diffuser space 5. The diffuser space 5 is defined by a single housing 7. The Nishida vaned diffuser suffers from the drawback of losses due to leakage between the vanes 2 and the housing wall 7 and between the diffuser ring 4 and the housing wall 7 with any thermal expansion of the diffuser space 5.
An example of another contemporary vaned diffuser is shown in U.S. Pat. No. 6,168,375 to LaRue. The LaRue vaned diffuser, as shown in FIG. 2, is spring-loaded with a flat wave spring and sealed with a sealing structure. The turbocharger 10 incorporates a compressor housing 12 having a volute 14 formed therein for receiving pressurized air through intake 18 from an air compressor impeller 16 rotatably disposed within the compressor housing 12. A vaned diffuser 20 is in the shape of an annular ring and is disposed within the compressor housing 12. The vaned diffuser 20 is positioned within a diffuser channel that is formed within an axially-facing surface of a compressor housing backplate 24. The vaned diffuser 20 comprises a plurality of vanes 26 that each project outwardly a distance away from an axially-facing vane diffuser surface. The vaned diffuser 20 has a tapered axially-facing surface moving radially from the impeller 16 to the volute 14 and a taper on a leading edge 30 of a vaneless section of the vane diffuser, as well as a taper on a trailing edge 32 of the vanes 26.
The LaRue turbocharger 10 has a wave spring 34 interposed between a backside surface 36 of the vaned diffuser 20 and a spring channel 38 that is formed within an axially-facing surface of the backplate 24. The spring 34 is a stamped or wire, metal, wave spring. The spring 34 is positioned between the vaned diffuser 20 and backplate 24 to impose a pressure load onto the vaned diffuser to urge it axially away from the backplate regardless of static pressure conditions within the compressor housing 12. This is done to keep the vanes 26 of the vaned diffuser 20 in contact with the compressor housing 12, at compressor housing end 28, as the compressor housing moves axially away from the backplate 24 under all turbocharger operating conditions. A pin 40 includes a first end that is placed within a pin slot 42 in the vane diffuser 20, and a second end that is placed within a pin slot 44 in the backplate 24. An annular O-ring seal 46 is disposed within a seal groove 50 formed along the axially-facing backplate surface 24, and is interposed between the vane diffuser and backplate to provide an air-tight seal therebetween. The O-ring seal 46 is intended to form and maintain an air-tight seal to prevent recirculation air flow around a backside surface of the vaned diffuser 20 even when the vaned diffuser 20 is moved away from the backplate surface 24.
The LaRue vaned diffuser suffers from the drawback of requiring two separate components or elements to perform the functions of pressure loading the vaned diffuser and sealing the vaned diffuser and backplate. The use of two such components adds complexity and cost. The use of two such components requires separate corresponding structure, e.g., spring channel 38 and seal groove 50, further adding complexity and cost.
Thus, there is a need for a system and method of restraining a vaned diffuser that reduces or eliminates performance losses. There is a further need for such a system and method that is cost effective and dependable. There is a further need for such a system and method that facilitates manufacture and assembly of the air boost device.